This invention relates to a method of making bipolar monolithic integrated circuits. It more particularly relates to a method of making such circuits by successive boron and phosphorus diffusions.
Bipolar monolithic integrated circuits are most often made in N-type silicon surfaces by successive diffusions of boron and phosphorus. The boron diffusion forms the middle layer of three layer devices, such as discrete transistors, in the integrated circuit. It is also used in reinforcing P-N junction isolation walls and in forming resistor units in the integrated circuit. The phosphorus diffusion forms an emitter region for discrete bipolar transistors in the integrated circuit. It is also used in forming resistors and interconnection paths. Ordinarily the boron is diffused identically into all surface portions where it is to form or reinforce a P-type doping region. Analogously, phosphorus is identically diffused into all surface portions where it is to reform or reinforce N-type doping. Boron diffuses to substantially the same depth in all regions, as does the phosphorus. Transistors, for example, all have about the same base width and correspondingly about the same current gain, regardless of power rating. Accordingly, the circuits must be designed with this limitation in mind.
It is appreciated that higher gain transistors can be selectively made in an integrated circuit by techniques already known in the prior art. For example, two base diffusions or two emitter diffusions could be used. Two different impurities of the same conductivity type, or two different surface concentrations of the same impurity can be used in the two diffusions. However, this entails an objectionable increase in the number and type of method steps that are needed. This can decrease yields and increase time and expense of manufacture. In addition, use of the lower surface concentration diffusion, to obtain a narrower base width, produces a lower doping level in the base region. A low doping level can cause undesirably high internal and contact resistance, and attendant electrical losses, in the base region. In view of such difficulties multiple diffusions are ordinarily avoided, the bipolar integrated circuits are normally made with transistors all having substantially the same current gain characteristics.
We have now found a simple and controllable technique for making some transistors in a boron-phosphorus diffused bipolar monolithic integrated circuit having a higher current gain. Gain on selected transistors can be increased in a simple and readily reproducible manner. The technique does not significantly decrease yields or significantly increase contact resistance, internal base resistance, or the time and expense of manufacture. Analogous advantages can be obtained when including pinch resistors in a bipolar monolithic integrated circuit.